No.4 a,b pakai cara ya

4.a
D = 315 cm = 31,5 dm
r = D/2 = 31,5/2 = 15,75

V = π.r².t
V = 22/7 . 15,75 . 15,75 . 12
V = 22 . 2,25 . 15,75 . 12
V = 9.355,5 dm³

b.
Berat = 9.355,5 dm³ x 9 kg
Berat = 84.199,5 kg